Various types of X-ray detectors are known, including varieties of gas-filled imaging detectors which are based on the conversion of X-ray photons in the gas volume to electrons and on the proportional amplification of the released photoelectrons in various wire electrode assemblies. Such detectors are described by J. E. Bateman in "Detectors for Condensed Matter Studies", Nuclear Instruments and Methods, A273 (1988) 721-730. Bateman also describes other X-ray photon detectors such as various solid scintillators and semiconductor devices.
There are also known gas scintillation detectors of various types as described by M. R. Sims, A. Peacock and B. G. Taylor, "The Gas Scintillation Proportional Counter", Nuclear Instruments and Methods, 221 (1984) 168-174. Various X-ray photon detectors are also described in U. W. Arndt, J. Appl. Cryst. 19 (1986) 145.
Gas-filled detectors are by far the most efficient and flexible X-ray detectors. They offer high localization resolution and good linearity, moderate-to-high counting rate capability, and a large variety of geometries over large active areas. However, gaseous (gas filled) detectors have the following disadvantages:
1. The X-ray to electron conversion in the gas causes a geometrical parallax error for photons impinging at an angular incidence.
2. The localization accuracy is limited, due to the relatively large range of photoelectron motion in the gas.
3. Space charge effects limit the counting rate.
4. The gas multiplication process in proportional detectors and the light production in gas scintillation detectors are relatively slow processes which limit the time resolution to between tens of nanoseconds and tens of microseconds.
5. A gas medium is not an efficient converter of energetic photons in the energy range of above about 10 KeV, even for high - Z Xenon gas.
Detectors having a relatively rapid response capable of operating at high X-ray flux are important in applications such as X-ray diffraction analysis in synchrotron radiation accelerators and X-ray radiography with intense X-ray generators. Fast detectors are also important when time correlated information is needed, as in the study of dynamic processes, as described in A. R. Faruqi, Nuclear Instruments and Methods, A273 (1988) 754.
Efficiency of X-ray detectors is exceedingly important, since any increase in efficiency enables X-ray dosages applied to subjects in therapeutic and diagnostic applications to be correspondingly reduced.
A state of the art X-ray detector for medical applications is described in Baru, S. E. et al, "Multiwire proportional chamber for a digital radiographic installation", Nuclear Instruments and Methods in Physics Research A283 (1989), pp. 431-435, the disclosure of which is incorporated herein by reference.
An X-ray detector for high flux operation is described in "A Novel Unidimensional Position Sensitive Multiwire Detector" by I. Dorion and M. Ruscev, IEEE Transactions on Nuclear Science, Vol. NS-34, No. 1, February 1987, pp. 442-448.
The inventors have published papers on imaging of photoelectrons using avalanche chambers including the following:
"High Accuracy Imaging of Single Photoelectrons by Low-Pressure Multistep Avalanche Chamber Coupled to a Solid Photocathode" by A. Breskin and R. Chechik, Nuclear Instruments and Methods in Physics Research 227, (1984) 24-28.
A. Breskin et al., "On the low pressure operation of multistep avalanche chambers" Nucl. Instrum. Methods, 220 349 (1984).
A. Breskin and R. Chechik, "Detection of single electrons and low ionization with low-pressure multistep chambers" IEEE Trans. Nucl. Sci. NS-32, 504 (1985).
R. Chechik and A. Breskin, "On the properties of low-pressure, TMAE-filled UV-photon detectors" Nucl. Instrum. Methods A264, 237 (1988).
A. Breskin et al., "A highly efficient low-pressure UV-RICH detector with optical avalanche recording" Nucl. Instrum. Methods, A273 (1988) 798.
P. Fischer et al., "Pad readout for gas detectors using 128-channel integrated preamplifiers" IEEE Trans. Nucl. Sci. NS-35, (1988) 432.
A. Breskin et al., "In beam performance of a low-pressure UV-RICH detector" IEEE Trans. Nucl. Sci., NS-35, (1988) 404.
A. Breskin et al., "Primary ionization cluster counting with low-pressure multistep detectors", IEEE Trans. Nucl. Sci., NS-36 (1989) 316.
S. Majewski et al., "Low-pressure Ultraviolet Photon Detector with TMAE Gas Photocathode", Nucl. Instrum. Methods, A264 (1988) 235.
V. Dangendorf et al., "An X-ray Imaging Scintillation Detector With Cs-I Wire Chamber UV-Photon Readout", WIS preprint 89-81 December-PH. Proceedings of the SPIE Conference on Instrumentation in Astronomy, Tucson, Feb. 1990.
The disclosures of these publications and of the reference cited therein are incorporated herein by reference.